The present invention relates to tension indicating devices and particularly to devices in the nature of washers which can be used to give a measure of the tension which has been induced in a bolt. Applicant, by this invention, has solved the problem of knowing when a bolt is no longer properly tensioned.
This invention is particularly, although not exclusively, useful in its application to bolts used in the assembly of towers for supporting electric transmission lines, for the support of rock formations in mines and tunnels and the assembly of structures such as bridges. The problem encountered and answered by the instant invention in regard to transmission towers is that the bolts used in the assembled tower are required to have a minimum amount of tension therein to insure proper assembly and safety to those persons and property in the immediate proximity of the tower. Obviously, when the bolts have lost their tension, in a tower of this type, an unsafe situation arises wherein collapse of the tower can jeopardize individuals and property. Another situation where the indicating washer is of particular value is in its use with bolts that maintain the stability of the rock formation in the walls and ceiling of mineshafts. Normal practice in the cutting of mineshafts is to stabilize the rock formation by using ceiling bolts. The problem encountered is that due to shifting of the rock formation because of blasting or of the tensioning of other bolts in the area, the tension in one or more of bolts in the area may be reduced and are no longer of value in safely maintaining the stability of the rock.
In both of the above noted situations it is mandatory that the tension in the bolts be monitored. In certain instances the monitoring of the safety of the mines or of the safety transmission tower construction is required by a Federal or State Agency and such constant and required monitoring can be a costly endeavor. In transmission towers the practice at this time is to assemble the tower and induce the requisite amount of tension in the bolt using a torque wrench. After construction has been completed the towers are required to be checked at given intervals to insure that the tension in the bolts is as required. This can be a time consuming and expensive process in that each bolt must be checked using a torque wrench.
In using ceiling stabilizing bolts in a mine it has been common to use a tension indicating type washer of the type shown and described in U.S. Pat. No. 3,329,058. A typical mine ceiling bolt is installed in a predrilled hole after it has been inserted through the aperture in the tension indicating device. The head of the bolt bears against the arched portion of the tension indicating device and is tensioned there against until a proper indication is given by the device. This device is costly to make and requires that there be a substantial amount of area around the bolt hole that is smooth and perpendicular to the axis of the bolt hole. This device also requires a close and difficult inspection in order to obtain the indication as to whether the measure of the tension which remains induced in the bolt is sufficient.